In a conventional, large steam turbine power plant, the high pressure turbine is constructed to receive steam through a number of nozzles that are arcuately spaced adjacent the periphery of the first stage of the turbine blading. These nozzles are segregated into groups; and an individual steam inlet or governor valve controls the steam flow through each nozzle group. In starting up the turbine, it is common practice to operate all the governor valves as a single valve to admit the steam through all of the nozzles equally, which is termed, "single valve" or "full arc" mode of operation. The single valve operation permits the heating of the rotor blades evenly to minimize thermal shock. When the turbine is "hot", and all of the governor valves are admitting the required steam in a partially open position, the efficiency of the plant is considerably reduced because of the throttling action or pressure drop across all the partially open valves.
To overcome such inefficiency after start-up, the steam inlet or governor valves are controlled to admit steam through a partial arc of nozzles sequentially; that is, for example, for a low steam flow demand or requirement, the steam is admitted through only a portion of fully open nonthrottled governor valves with only one valve or group of valves being in a throttling position to control any variation in steam flow demand. The remaining valves are closed. As the steam flow demand increases, for example, the valve or valve group in the throttling position opens fully; and then, the next valve or group of valves opens as required to control the steam flow variation in a predetermined sequence. For a decreasing steam flow demand, the valves are controlled in a reverse sequence toward their closed position. Thus, any throttling inefficiency occurs for only one valve or group or valves for certain steam flow requirements.
A conventional steam inlet or governor valve nozzle arrangement has non-linear characteristics after it reaches a certain percentage of total lift; that is, for a valve with a total lift of 71/2 inches, for example, approximately 90 to 95 percent of its total maximum steam admission or flow capability occurs in the first two to three inches of valve opening, with the remaining five to ten percent occurring in the remaining 4 to 5 inches of valve travel.
Further, the point at which the valve changes from a linear characteristic to a non-linear characteristic, or in other words, the critical flow point, referred to herein as the FC point of the valve, changes in accordance with the total turbine steam flow, because of the pressure drop across the nozzle group controlled by the valve. For example, if the total steam flow is above, say, 64% of rated flow, the FC point varies from, say, about 3 inches of valve lift down to about 2 inches, as the steam flow increases.
The lift above the FC flow point of the valve, that is, where a slight change in steam flow results in a substantial change in valve lift, referred to as the high slope region, is an undesirable operating region because it causes excessive wear on the valve due to slight variations in steam flow requirements. The linear region of the curve below the FC point of the valve provides stable control, but reduces the efficiency of the plant because of throttling losses. Thus, the optimum position for each of the valves not in a fully open or closed position is the FC point.
In order to overcome the disadvantages of operation in the high slope region, U.S. Pat. No. 3,878,401 dated Apr. 15, 1975, discloses a system, for example, where an increase in steam flow demand which would ordinarily position the steam inlet valve in the non-linear or high slope region, instead, opens the next sequential valve or group of valves, into the linear region. The preceding valve in the sequence is moved toward a closed position and into the linear region to compensate for the flow through the next sequential valve. This overlap system effectively provides stability of operation with minimum valve wear for sequential valve operation.
In order to provide minimum throttling losses and maximum stability for all levels of steam flow when operating in the sequential valve mode, none of the governor valves should be in any position other than fully open, fully closed, or at the end of its linear operating region, referred to as the FC point. For example, at a lower level of load demand, the first valve sequence or group should be either fully open or at the FC point, with the subsequent valves in the sequence fully closed. At a higher level, the first valve operating sequence should be fully open, the next sequence at the FC point, and the remaining sequence fully closed. At still a higher level, first sequential operating groups should be fully open with the last valve or group at the FC point. At any steam flow where one or more valves are partially open below the FC point, the plant is subjected to steam throttling losses; and at flow where one or more of the valves is partially open above the FC point, there may be instablity in response to small steam flow variational requirements and excessive wear.
The digital electrohydraulic turbine power plant control system disclosed in the referenced application to Giras et al. includes indicators on the control panel which inform the operator of the actual position of the governor valves. However, because of the dynamic variation in the location of the FC point at various levels of steam flow and pressure, the operator cannot with certainty determine that the plant is operating with the valves at their optimum position unless, of course, they are all fully open.
In view of the foregoing, it is desirable that a system be provided, such that the operator of a turbine power plant is able to determine when the valves are positioned at their optimum position for all levels of load such that the load, steam pressure, or flow may be varied within operational constraints to operate the plant with a minimum of throttling losses. For use in a system of the type disclosed in the Giras application that does not include the overlap capability of the referenced U.S. patent, the system should also detect when one or more of the valves are in the non-linear or high gain position above the FC point.